1. Field of the Invention
The present invention relates to a pre-ignition estimation/control device for an internal combustion engine, which is installed on a vehicle or the like and can prevent pre-ignition.
2. Description of the Related Art
Pre-ignition causing combustion noises and fluctuations in output may occur in a high-compression-ratio spark-ignition internal combustion engine. The pre-ignition refers to a phenomenon of an excessively large fluctuation in pressure inside a combustion chamber of an internal combustion engine, which results from combustion. Pre-ignition which occurs before the spark ignition is generally known. In addition to this type of pre-ignition, there are phenomena including pre-ignition in which a mixture (mixed gas of the air and a fuel) in the combustion chamber reaches high temperature by compression and is auto-ignited, and pre-ignition in which a deposit or the like at a tip of a spark plug forms a hot spot and causes ignition. Moreover, there is known a knock in which an end gas at a periphery of the combustion chamber is auto-ignited in the combustion process after the ignition.
These phenomena cause noises and vibrations as well as damages in the combustion chamber of the internal combustion engine, and the internal combustion engine may finally become no longer operable.
To address this, methods of estimating occurrence of pre-ignition based on environmental conditions and operation conditions, thereby preventing the pre-ignition, have conventionally been proposed.
In a conventional method of preventing the pre-ignition, pre-ignition, which occurs at low rotation speed, under a high load, and at high temperature, is determined based on the temperature and operation states of the internal combustion engine, and an effective compression ratio is reduced as the temperature increases, thereby preventing the pre-ignition (refer to Japanese Patent Application Laid-open No. 2005-76466, for example).
Moreover, in another conventional method of preventing the pre-ignition, the octane number of a fuel is estimated, and a likelihood of occurrence of the pre-ignition is determined based on the estimated octane number of the fuel, thereby preventing the pre-ignition on startup by decreasing the effective compression ratio on the startup according to the environmental temperature of the internal combustion engine (refer to Japanese Patent Application Laid-open No. 2009-114973, for example).
Japanese Patent Application Laid-open No. 2005-76466 describes that the pre-ignition is prevented by decreasing the effective compression ratio according to the increase in the temperature of the internal combustion engine. However, causes of the occurrence of the pre-ignition include fuel properties in addition to the temperature in the internal combustion engine and the increase in temperature caused by the compression in the combustion chamber, and it is known that the pre-ignition occurs more frequently as the octane number of the fuel becomes low.
In view of the above, even in a case where the environmental temperature (temperature of an intake air to be introduced into the combustion chamber as an ambient temperature around the internal combustion engine, for example) is low, if the octane number of the fuel is low, pre-ignition possibly occurs. The opposite holds true, and even if the octane number is high, high environmental temperature possibly causes pre-ignition.
Japanese Patent Application Laid-open No. 2009-114973 describes that a likelihood of occurrence of the pre-ignition is determined, and the pre-ignition on startup is prevented by setting the effective compression ratio on the startup according to the determination. However, Japanese Patent Application Laid-open No. 2009-114973 describes only the prevention of the pre-ignition that occurs on the startup (prevention of the pre-ignition refers to prevention of such an operation state that possibly causes the pre-ignition), and does not consider pre-ignition which occurs “after the startup”. Moreover, Japanese Patent Application Laid-open No. 2009-114973 does not consider a method of avoiding pre-ignition after the preventive operation of the pre-ignition (avoidance of pre-ignition refers to a change of an operation state to prevent pre-ignition from occurring any more when pre-ignition actually occurs). In addition, though it is assumed that the likelihood of the occurrence of the pre-ignition depends on the intake air temperature, the coolant temperature, and the octane number of the fuel, which are considered independently, the likelihood of the occurrence of the pre-ignition is not considered under a combined condition thereof. There are a plurality of conditions which cause the pre-ignition more frequently, and it is necessary to consider the respective conditions, thereby producing an index of the likelihood of the pre-ignition.
The method described in Japanese Patent Application Laid-open No. 2009-114973 describes that the likelihood of the pre-ignition is determined according to the octane number. When a fuel having a high octane number is used, at whatever high intake air temperature and whatever high water temperature the startup may be conducted (hot start at 50° C. of the intake air temperature and 110° C. of the water temperature in a desert, for example), control of reducing the effective compression ratio is not provided. However, even if a fuel having a high octane number (high octane fuel of 95 [RON], for example) is used, it is experimentally known that pre-ignition may occur at high intake air temperature and high water temperature.